SYNOPSIS

DESCRIPTION

A process's CPU affinity mask determines the set of CPUs on which it is
eligible to run. On a multiprocessor system, setting the CPU affinity
mask can be used to obtain performance benefits. For example, by
dedicating one CPU to a particular process (i.e., setting the affinity
mask of that process to specify a single CPU, and setting the affinity
mask of all other processes to exclude that CPU), it is possible to
ensure maximum execution speed for that process. Restricting a process
to run on a single CPU also avoids the performance cost caused by the
cache invalidation that occurs when a process ceases to execute on one
CPU and then recommences execution on a different CPU.
A CPU affinity mask is represented by the cpu_set_t structure, a "CPU
set", pointed to by mask. A set of macros for manipulating CPU sets is
described in CPU_SET(3).
sched_setaffinity() sets the CPU affinity mask of the process whose ID
is pid to the value specified by mask. If pid is zero, then the
calling process is used. The argument cpusetsize is the length (in
bytes) of the data pointed to by mask. Normally this argument would be
specified as sizeof(cpu_set_t).
If the process specified by pid is not currently running on one of the
CPUs specified in mask, then that process is migrated to one of the
CPUs specified in mask.
sched_getaffinity() writes the affinity mask of the process whose ID is
pid into the cpu_set_t structure pointed to by mask. The cpusetsize
argument specifies the size (in bytes) of mask. If pid is zero, then
the mask of the calling process is returned.

RETURNVALUE

On success, sched_setaffinity() and sched_getaffinity() return 0. On
error, -1 is returned, and errno is set appropriately.

ERRORS

EFAULT A supplied memory address was invalid.
EINVAL The affinity bit mask mask contains no processors that are
currently physically on the system and permitted to the process
according to any restrictions that may be imposed by the
"cpuset" mechanism described in cpuset(7).
EINVAL (sched_getaffinity() and, in kernels before 2.6.9,
sched_setaffinity()) cpusetsize is smaller than the size of the
affinity mask used by the kernel.
EPERM (sched_setaffinity()) The calling process does not have
appropriate privileges. The caller needs an effective user ID
equal to the real user ID or effective user ID of the process
identified by pid, or it must possess the CAP_SYS_NICE
capability.
ESRCH The process whose ID is pid could not be found.

VERSIONS

The CPU affinity system calls were introduced in Linux kernel 2.5.8.
The system call wrappers were introduced in glibc 2.3. Initially, the
glibc interfaces included a cpusetsize argument, typed as unsignedint.
In glibc 2.3.3, the cpusetsize argument was removed, but was then
restored in glibc 2.3.4, with type size_t.

CONFORMINGTO

These system calls are Linux-specific.

NOTES

After a call to sched_setaffinity(), the set of CPUs on which the
process will actually run is the intersection of the set specified in
the mask argument and the set of CPUs actually present on the system.
The system may further restrict the set of CPUs on which the process
runs if the "cpuset" mechanism described in cpuset(7) is being used.
These restrictions on the actual set of CPUs on which the process will
run are silently imposed by the kernel.
sched_setscheduler(2) has a description of the Linux scheduling scheme.
The affinity mask is actually a per-thread attribute that can be
adjusted independently for each of the threads in a thread group. The
value returned from a call to gettid(2) can be passed in the argument
pid. Specifying pid as 0 will set the attribute for the calling
thread, and passing the value returned from a call to getpid(2) will
set the attribute for the main thread of the thread group. (If you are
using the POSIX threads API, then use pthread_setaffinity_np(3) instead
of sched_setaffinity().)
A child created via fork(2) inherits its parent's CPU affinity mask.
The affinity mask is preserved across an execve(2).
This manual page describes the glibc interface for the CPU affinity
calls. The actual system call interface is slightly different, with
the mask being typed as unsignedlong*, reflecting the fact that the
underlying implementation of CPU sets is a simple bit mask. On
success, the raw sched_getaffinity() system call returns the size (in
bytes) of the cpumask_t data type that is used internally by the kernel
to represent the CPU set bit mask.
The cpu_set_t affinity mask size provided by glibc only allows for upto
1024 CPUs. It is possible to build Linux kernels with greater than 1024
CPUs. Any application using the statically sized cpu_set_t will fail
with EINVAL on such kernels. It is thus recommended that applications
avoid using the statically sized cpu_set_t type, and instead
dynamically allocate a mask using the CPU_*_S macros described in the
CPU_SET(3) man page. Since it is not possible to determine ahead of
time what NR_CPUS value the kernel was built with, applications must be
prepared to catch EINVAL, and retry the command with a larger
dynamically allocated mask. The example that follows illustrates
portable usage.